It is common practice today to store many gases liquids or vapors such as gasoline in underground storage tanks for safety reasons as well as space saving reasons. The most common type of installation presently in use involves a single shell steel tank which must be encased in 12 inches of concrete.
The installation procedure is rather complex, time consuming and could be dangerous to the workers during the actual installation procedures. With a common type size tank, such as for storage of gasoline, the contractor excavates to a depth of approximately 10 feet by using a backhoe. The excavation is made rectangular in shape. He must have two or three laborers enter the excavation and install 4 foot by 8 foot wood shoring around the sides of the excavation to prevent collapse and to make a form for pouring a concrete footing. The contractor then pours a 12 inch concrete base and the laborers must again enter the excavation to evenly distribute the concrete.
After the concrete hardens, the contractor must lift the tank into place in the excavation and install piping. The tank and piping are encased in 12 inches of poured concrete up to the level of the top of the tank after successfully testing the tank and piping hydrostatically at 30 psi for 30 minutes. Load bearing blocks are then placed around the tank perimeter, clean sand is placed over the tank and a reinforced concrete top slab is poured to grade level.
There are certain difficulties and potential hazards that can develop from an installation of the above type. For example, the excavation could collapse with or without shoring causing injury or death to laborers in the excavation. Furthermore, the tanks are single shell steel tanks which ultimately corrode and allow their contents to leak out resulting in an extremely dangerous condition. Fuels have been known to migrate underground and enter sewer lines, basements, underground utility lines, contaminate reservoirs and cause explosions. Also, during concrete encasement, the tank can float causing damage to the installation. Finally, it should also be kept in mind that the excavation remains open until the piping is completed and arrangements are made to encase the tank. This may take two weeks time during which pedestrians, children, workers, or even vehicles can fall into the excavation. In its entirety, the above manner of installation requires approximately four weeks to complete the work.
It is readily apparent from the potential disadvantages and difficulties encountered with the system in use that there is considerable room for improvement in the field of underground storage tank installation.